Fluid catalytic cracking involves the catalytic breakdown of heavy hydrocarbons to lighter fuels and petrochemical stocks. A typical catalytic cracking (FCC) unit has a hydrocarbon cracking zone and a catalyst regeneration zone. A particulate catalyst, generally containing a zeolite, is cycled between the cracking zone where the hydrocarbon feedstock is cracked and a regeneration zone where the coke formed on the catalyst in the cracking zone is burned off. The combustion gases formed in the regeneration zone will usually contain some noxious gases which are considered air pollutants. For example, carbon monoxide is generally formed during oxidation of the coke. See U.S. Pat. No. 3,909,392. In addition, when the hydrocarbon feedstocks contain sulfur and/or nitrogen, the combustion gases will usually contain the oxides of these elements in various amounts.
The carbon monoxide present in the combustion gas may be controlled by adding small amounts of a carbon monoxide oxidation promoter, such as platinum. See U.S. Pat. No. 4,072,600. The sulfur oxides may be controlled by including a sulfur sorbent, such as alumina, in the circulating inventory, i.e., in admixture with the cracking catalyst. See U.S. Pat. No. 4,071,436. It has also been found that inclusion of a sulfur dioxide oxidation promoter enhances the removal of the sulfur from the combustion gas by the sulfur sorbent. See U.S. Pat. No. 4,115,250. Hitherto, one of the most effective sulfur dioxide oxidation promoters was platinum. Although effective, this promoter has the disadvantage of increasing the amount of the oxides of nitrogen present in the flue gas. As noted above, platinum can also serve as a carbon monoxide oxidation promoter. It would be desirable to use an oxidation promoter in the regeneration process which retains the effectiveness of platinum in promoting the oxidation of carbon monoxide and sulfur dioxide without contributing to the formation of nitrogen oxides.